The proposed experiments are designed to investigate the role of metabolically related vasoactive chemicals and their interactions in the local control of micro-vascular hemodynamics in skeletal muscle. The preparation will be the open cremaster muscle of the rat, suffused with a Ringer's solution whose component can be controlled and altered as desired. Measurements of red cell velocity (V) will be made in arterioles, capillaries, and venules by an on-line and video, off-line dual-slit photometric method. Arteriolar blood flow (Q) will be calculated from the equation Q equals V pi D2/6.4, where D is the internal diameter of the arteriole measured from video tapes. Capillary density will also be determined from the video tapes. The pO2 of the tissue and the fluid suffusing the tissue will be measured with the Whalen recessed oxygen microelectrode. These techniques will be used to quantitate the role of oxygen in the production of post-occlusive reactive hyperemia, to measure the interaction between changes in pO2 and extracellular pH on the microvessels, to study the interaction of pO2 in the control of the skeletal muscle circulation, and to determine the interaction of CO2 with intracellular and extracellular pH, thus clarifying the mechanisms of hypercapnic vasodilation. The data will be analyzed for linear as well as nonlinear combinations, such as mutual potentiations or attenuations. The results of these experiments will allow the individual effects of these various vasoactive chemicals to be combined into a single function describing the blood flow to skeletal muscle and its distribution within the muscle.